1. Field of the Invention
The present invention relates to a classifying apparatus and a classifying method which are used to produce dry toner for developing electrostatic images in electrophotography, electrostatic recording, electrostatic printing, etc.; and to a toner and a method for producing the toner.
2. Description of the Related Art
Several traditional approaches are known for classifying pulverized coarse toner particles: a combination of a single classifier BZ1 and a single pulverizer FZ1 (as shown in FIG. 1, for example); a combination of two classifiers BZ1 and BZ2 and a single pulverizer FZ1 (as shown in FIG. 2, for example); and a combination of two classifiers BZ1 and BZ2 and two pulverizers FZ1 and FZ2 (as shown in FIG. 3, for example). Note that, in FIGS. 1 to 3, reference character A denotes a fine powder-classifying unit (step).
One type of the pulverizers used in these systems is a jet pulverizer that propels raw particles in a high-pressure air stream spouted from a jet nozzle to cause the particles to collide with each other or hit a wall or other objects and thus pulverize the particles.
The jet pulverizer will be described with reference to FIG. 3.
In FIG. 3, raw materials are fed through a feed pipe FE1, and together with the previously pulverized product and high-pressure air, introduced into a first classifier BZ1 where they are classified into coarse powder and fine powder.
The coarse powder is pulverized in a first pulverizer FZ1 and collected once in a cyclone CY1. The collected powder is introduced into a second classifier BZ2 where it is classified again into coarse powder and fine powder.
The thus-classified coarse powder is then pulverized in a second pulverizer FZ2 and collected in a cyclone CY2.
The collected powder is sent to a fine powder-classifying unit where it is classified into fine powder and a final product.
In this jet pulverizer, however, the powder fed to the first pulverizer BZ1 contains not only the raw powder but also particles of various sizes that are in the process of pulverization. Thus, the jet pulverizer is low in classification efficiency, which is problematic.
FIG. 4 shows the configuration of an air classifying apparatus (a DS air classifying apparatus) that is used as the pulverizer BZ1 or BZ2.
The air classifying apparatus includes a dispersion chamber (or collector dispersion chamber) 1, a classification chamber 7 and a bottom hopper 8.
A powder material feeding port 2 for feeding a primary air stream and powder material is connected with the dispersion chamber 1 at the upper periphery as a flow inlet at the circumferential surface of a cylindrical casing 15.
An umbrella-shaped center core 9 is disposed within the dispersion chamber 1 near its bottom. Further, an umbrella-shaped separator core 13 is disposed below the center core 9. A slatted secondary air stream inlet 14 (also referred to as “louver”) is disposed about the classification chamber 7 along the outer periphery thereof to facilitate dispersion of the powder materials and accelerate the swirling of the powder materials.
In this manner, the fine powder within the classification chamber 7 is guided to a fine powder discharge port 10 provided in the separator core 13 and discharged through a fine powder discharge pipe 11 connected to the fine powder discharge port 10 by the suction force provided by a blower.
On the other hand, the coarse powder is discharged from an annular coarse powder discharge port 12 provided along the outer periphery of the lower edge of the separator core 13.
A typical DS air classifier operates by the principle that centrifugal and centripetal forces of different magnitudes act on the coarse particles and fine particles present in a powder material as the secondary air stream flows into the classification chamber and causes a non-free flow of the swirling particles.
For this reason, it is desirable that the particles dispersed in the classification chamber be quickly classified into coarse particles and fine particles without allowing the particles to re-aggregate together.
However, conventional DS air classifying apparatuses are now required to disperse an increased number of particles because toner particles are becoming increasingly small and pulverization performance of pulverizers has improved significantly. When used to disperse such increased number of particles, the dispersion performance of conventional DS air classifying apparatuses will decrease, resulting in decreased classification accuracy. This inevitably leads to inclusion of coarse particles into a fine powder discharge region. As a result, the product obtained by the classification process may cause background smear and improper transfer and may therefore lead to decreased image quality.
Also, such inclusion of coarse particles may also impose an excessive load on the classifier during the production process and may thus decrease the efficiency of classification as well as the energy efficiency of pulverization.
Japanese Patent (JP-B) No. 2766790 or other documents disclose a classifier in which a louver is provided in a dispersion chamber (collector).
In this classifier, a nozzle is inserted in the louver for introducing powder and primary air. Secondary air is introduced from the outer periphery of the louver to facilitate the dispersion of the powder. This configuration is disadvantageous in that when raw materials are fed with high-pressure air, the pressure difference within the dispersion chamber causes the raw materials to be released from the dispersion chamber into the atmosphere, making it difficult to further continue to conduct the classification process.
Also, Japanese Patent Application Laid-Open (JP-A) No. 2009-189980 discloses an air classifier including a louver ring having a plurality of guide slats annularly arranged at regular intervals in a dispersion chamber, and a flow path which encircles the louver ring and receives high-pressure air and powder material fed from a powder material feeding port, wherein ultrafine powder generated through pulverization is collected in advance in the dispersion chamber to increase classification accuracy and wherein the high-pressure air and raw material are passed through the gaps between the slats of the louver ring disposed inside the dispersion chamber to a collector dispersion chamber thereby improving dispersibility. Use of this air classifier allows the powder material fed from the powder material feeding port to pass through the gaps between the slats of the louver ring, whereby it can be fed to the dispersion chamber from the entire circumferential positions. The above air classifier shows an advantageous effect of preventing aggregation of the particles as compared with conventional classifiers.
However, since part of the louver ring is located across an extended line of the louver ring side wall (i.e., an extended line of a straight line connecting a powder material feeding port's inner inlet and a powder material feeding port's inner outlet) (FIG. 5), in the above classifier, air flow fed from the powder material feeding port collide with the slats to potentially be slow in swirling speed. In addition, as a result of the collision of the airflow with the slats, the airflow through the gaps between the slats is disturbed, and the speed of the airflow through the gaps therebetween is varied from place to place in the annually arranged slats. Thus, the fed powder material is not sufficiently dispersed to potentially lead to a drop in classification accuracy and production yield, which is problematic.
Also, JP-B No. 2597794 or other documents disclose a technique in which after charged through a raw material feeding pipe, raw material (toner) is dispersed by gas introduced from a guide vane of a dispersion chamber.
However, this proposed technique poses a problem that the fed raw material cannot efficiently be dispersed since both of the raw material and the gas do not pass through the louver ring.